The resistance value of an AC motor varies according to temperature. If the resistance value varies, and an error is produced between the resistance value configured on the control side and the actual resistance value, a desired output torque may not be obtained in some cases, and desired control response may not be obtained in some cases. Accordingly, it is necessary to accurately compute the resistance value of an AC motor.
With the technology disclosed in Patent Literature 1, a direct current (DC) voltage is applied and the current value is measured in a time of set length immediately after activation of an induction motor, and the variations of a primary resistance value and a secondary resistance value are computed on the basis of the measured current value and the current value at a standard temperature. With the induction motor control device disclosed in Patent Literature 2, test power is supplied to an inverter, the inverter is given a voltage command at a frequency so that a polyphase induction motor does not start to revolve, and a secondary resistance value is computed on the basis of a primary resistance value of the polyphase induction motor that is measured in advance.
With the technology disclosed in Patent Literature 1 and 2, a determination of whether or not the AC motor is revolving is not conducted. If the AC motor is revolving, the AC motor has complex impedance characteristics depending on the rotational frequency, and it is difficult to estimate a primary resistance value and a secondary resistance value from the response characteristics of the current value with respect to the voltage applied to the AC motor. For this reason, if the AC motor is revolving, accurately computing the primary resistance value and the secondary resistance value of the AC motor is difficult. Accordingly, the power conversion device disclosed in Patent Literature 3 computes the angular velocity of an AC motor on the basis of a drive command as well as the voltage and the current in a rotating coordinate system that rotates in synchronization with a rotating magnetic field produced in the AC motor, and computes the resistance value of the AC motor if the angular velocity is 0.